Collimator for Medical Imaging and Fabrication Method

ABSTRACT

A photon collimator, suitable for use in medical imaging equipment, is constructed from a block of photon-attenuating material, such as solid tungsten or molybdenum alloy that defines a plurality of integrally formed septa slats. Each slat has an elongated length dimension greater than thickness and depth dimensions, and is oriented in an opposed pattern array that is laterally spaced relative to its respective thickness dimension. An aperture channel is defined between each pair of opposed slats. Rows of integrally formed slats in one block or separately affixed blocks may be stacked on each other at skewed angles to form two-dimensional grids of apertures having polygonal cross sections. The slats may be formed by electric discharge or laser thermal ablation machining, such as by a sequential passing of an EDM wire cutting head along the pattern array, repeating sequential cutting of respective channel depth and width.

CLAIM TO PRIORITY

This application claims the benefit of co-pending United Statesprovisional patent application entitled “Crossed-Slat Collimator Formedby Wire-EDM Process”, filed Sep. 26, 2011 and assigned Ser. No.61/539,015, which is incorporated by reference herein.

BACKGROUND OF THE DISCLOSURE

1. Field of the Invention

The invention relates to photon collimators, and more particularlycollimators suitable for medical imaging apparatus that are fabricatedwith electric discharge machining (EDM) techniques or other thermalablation cutting techniques, such as laser cutting.

2. Description of the Prior Art

Photon collimators are utilized in medical imaging and therapyapparatus, such as gamma cameras, to allow passage of photons that havetrajectory paths aligned with apertures that are defined by thecollimator structure. Photons having non-aligned trajectory paths areblocked by the collimator structure. Known collimators are shown inFIGS. 1 and 2, and are often fabricated from relatively densematerial-commonly lead alloys. FIG. 1 depicts a known type of cast leadalloy collimator 20, having a matrix-like array of cast-in-placeaperture through holes 22 that are aligned along respective length andwidth axes 24, 26. The apertures 22 are often formed by casting moltenlead around a matrix grid of mold pins (not shown).

FIG. 2 depicts another known type of fabricated lead alloy collimator 30having a matrix-like array of aperture through holes 32 that is formedfrom a repetitive pattern of opposing lead foil strips 34, 36 that arebonded together with a layer of glue 38. Each opposed face of the foilstrips 34, 36 is calendered (i.e., compressed or squeezed) with a seriesof half-polygonal (e.g., semi-circular or half-hexagonal) impressionsthat when joined together in opposed fashion along the glue layer 38form each individual aperture 32. Adjoining pairs of lead strips 34, 36are in turn bonded with glue layers 38 to form a unitized, fabricatedcollimator structure 30.

Some jurisdictions are discouraging use of lead components in general,including medical equipment. Hence, there is a perceived need to replacelead alloys in medical and other equipment collimators with substitutedense alloy materials. Tungsten (W) and Molybdenum (Mo) alloys are beingconsidered as lead substitutes in collimators, but their hardness andrelatively high melting temperatures make them more difficult tofabricate for collimators. Some low energy-level collimators have beenfabricated from molybdenum foil, but the foil thickness is too thin forthe photon energy levels and density normally required for human medicalimaging apparatus. Some attempts have been made to fabricate smallercollimators not suitable for human medical imaging by laser sinteringmolybdenum and tungsten powders. However, laser sintering complex gridpatterns of the size necessary for medical imaging photon collimators isrelatively time consuming and expensive.

Tungsten and molybdenum alloys are not as easily cast as lead to meethigh precision tolerances required for medical imaging collimators—oftenrequiring less than 1/20 degree variation between adjoining apertures.Due to material hardness properties, tungsten alloys are not readilycalendered in precision foil strips of sufficient thickness for medicalimaging collimators, or readily mechanically machined (e.g., by drillingor milling). It is difficult to maintain inter-aperture size, shape andspacing variation within acceptable tolerances by mechanical machiningtechniques. While some relatively small collimators for less than humansize imaging apparatus have been fabricated by EDM cutting individualapertures, a typical gamma camera collimator for human patients requiresfabrication of thousands of aperture holes in a precision matrix-likegrid. Mechanically machining or thermal ablation cutting such a largequantity of individual apertures is time consuming and costly.

Thus, a need exists in the art for a photon collimator that can befabricated with inter-aperture size, shape and/or spacing variationtolerances required for human-sized medical imaging equipment.

Another need exists in the art for a non-lead alloy photon collimatorthat can be fabricated from tungsten, molybdenum or otherphoton-attenuating alloys with known, cost-effective machiningtechniques and equipment.

An additional need exists in the art for a photon collimator that can befabricated with fewer forming operations otherwise required to fabricatea matrix-like grid of individual aperture holes on a one-by-one basis.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to create a photoncollimator that can be fabricated from tungsten, molybdenum or otherphoton-attenuating alloys, and meet the aperture variation tolerancesrequired for human-sized medical imaging equipment.

Another object of the present invention is to create a photon collimatorthat can be fabricated from tungsten, molybdenum and other non-leadalloys, with known, cost-effective fabrication techniques and equipment.

An additional object of the present invention is to create a non-leadalloy photon collimator that can be fabricated with fewer formingoperations otherwise required to fabricate a matrix-like grid ofindividual aperture holes on a one-by-one basis.

These and other objects are achieved in accordance with the presentinvention by a photon collimator, suitable for use in medical imaging orother equipment, that is constructed from a photon-attenuating materialthat defines a plurality of integrally formed septa slats. Each slat hasan elongated length dimension greater than thickness and depthdimensions, and is oriented in an opposed pattern array that islaterally spaced relative to its respective thickness dimension. Aone-dimensional (i.e., generally linear) aperture channel is definedbetween each pair of opposed slats. Rows of integrally formed slats andone-dimensional apertures in one collimator block or separately affixedcollimator blocks may be stacked on each other at skewed angles to formgrids of two-dimensional apertures having polygonal cross sections. Thestacking of septa slats may be accomplished by machining layers of slatswithin a single block and/or stacking and affixing separate blocks ontop of one another. The slats may be formed by electric discharge orlaser thermal ablation machining, such as by a continuous pass of an EDMwire cutting head along the pattern array, repeating sequential cuttingof respective channel depth and width, or by laser sintering the slatsand their related support structure.

The present invention features a method for making a photon collimator,comprising cutting a plurality of elongated aperture channels through ablock of photon-attenuating material by thermal ablation machiningselected from the group consisting of electric discharge machining andlaser machining. The respective channels have an elongated lengthdimension greater than width and depth dimensions and are oriented in apattern array that is laterally spaced relative to their respectivewidth dimensions, thereby leaving a pattern of opposed septa slatsdefining width and length dimensional boundaries of the aperturechannels.

The present invention also features a photon collimator, comprising ablock of photon-attenuating material defining a plurality of integrallyformed septa slats. Each slat has an elongated length dimension greaterthan thickness and depth dimensions, and is oriented in an opposedpattern array that is laterally spaced relative to its respectivethickness dimension. An aperture channel is defined between each pair ofopposed slats.

The present invention additionally features a photon collimator that isconstructed from a block of photon-attenuating material, defining firstand second pluralities of integrally formed planar septa slats onopposite sides of the block that are skewed in relative alignment witheach other. Each slat in each respective plurality of slats has anelongated length dimension greater than its thickness and depthdimensions, and is oriented in an opposed pattern array that islaterally spaced relative to its thickness dimension. The collimatoralso has respective first and second aperture channels between eachcorresponding pair of respective opposing first and second pluralitiesof slats. The respective first and second aperture channels are incommunication with each other and form a pattern of polygonalcross-section through-apertures.

The objects and features of the present invention may be applied jointlyor severally in any combination or sub-combination by those skilled inthe art.

BRIEF DESCRIPTION OF THE DRAWINGS

The teachings of the present invention can be readily understood byconsidering the following detailed description in conjunction with theaccompanying drawings, in which:

FIG. 1 is a plan view of a known cast lead collimator;

FIG. 2 is a plan view of a known fabricated lead foil collimator;

FIG. 3 is a perspective view of an embodiment of a one-dimensional slatcollimator of the present invention;

FIG. 4 is a cross sectional view of the collimator of FIG. 3;

FIG. 5 is an plan view of the collimator of FIG. 3;

FIG. 6 is a schematic elevational view showing an embodiment of an EDMcutting sequence for formation of plural septa plates;

FIG. 7 is a perspective view of an embodiment of a two-dimensional slatcollimator of the present invention, having first and second arrays ofskewed, stacked septa plates;

FIG. 8 is a front elevational view of the collimator of FIG. 7;

FIG. 9 is a side elevational view of the collimator of FIG. 7;

FIG. 10 is a top plan view of the collimator of FIG. 7;

FIG. 11 is a front elevational view of a variable-focus collimatorembodiment of the present invention;

FIG. 12 a side elevational view of the variable-focus collimatorembodiment FIG. 11;

FIG. 13 is an exploded view of a stacked collimator embodiment of thepresent invention;

FIG. 14 is a top plan view of the collimator of FIG. 13; and

FIG. 15 is a top plan view of another collimator embodiment of thepresent invention.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures.

DETAILED DESCRIPTION

After considering the following description, those skilled in the artwill clearly realize that the teachings of my invention can be readilyutilized in the fabrication of photon collimators, comprising a block ofphoton-attenuating material, including lead or non-lead alloys, such asby way of non-limiting example tungsten or molybdenum alloys, defining aplurality of integrally formed septa slats, each slat having anelongated length dimension greater than its thickness and depthdimensions, and oriented in an opposed pattern array that is laterallyspaced relative to its thickness dimension; and an aperture channeldefined between each pair of opposed slats. The slats are fabricated bycutting the elongated aperture channels with EDM or laser thermalablation machining. The slats and supporting structure may also befabricated by laser sintering non-lead alloy powders, such as powderscomprising tungsten or molybdenum alloy powders.

FIGS. 3-5 show an embodiment of a one-dimensional collimator 40 that isconstructed in accordance with the teachings of the present invention.The collimator 40 is fabricated from a block of photon-attenuatingmaterial, such as a solid block of tungsten or molybdenum alloy, orlaser sintered from powders comprising those alloys. The collimator 40has an array of elongated, aligned one-dimensional (i.e., generallylinear) aperture channels 41 of dimensional width W (here shown havingrectangular projection cross-sectional profile A) that is definedbetween septa slats 42. Each slat 42 has a top face 44 defining a slatthickness T and side faces 46 that define slat length L and depth D. Thesepta slats 42 are shown schematically and are not drawn to scale. Forexample, slat 42 may have a thickness T of 0.125-2.0 mm (0.005-0.078in), depth D of 24-50 mm (0.45-2.0 in), and an aperture width W spacingbetween slats of 1.0-1.5mm (0.039-0.138 in). A typical collimator 40 mayhave an overall slat 42 length L of 400 mm (15 in) and have a slat arrayoverall width of 600 mm (21 in). Thus a typical collimator 40 may haveover 300 machined slats in an array. The slats 42 are affixed inrelative position to each other by machined in place rails 48 that areoriented laterally relative to the slats.

As shown in FIG. 6, the collimator 40 slats 42 are formed within ablock, such as tungsten or molybdenum alloys, by moving a heat ablationcutting tool, such as a laser or EDM head in a pathway shownschematically as dash-dot line 52. Alternatively the material in theprofile shown in the dash-dot line may be removed by plunge-cutting witha laser or EDM head in the depth D dimension. The removed material formsthe channel slots 41. A through-passage is completed between the channelslots 41, to create the apertures A, by cutting the back side of theblock opposite the slats 42, and also forming the rails 48. Thus in asingle cutting pass sequence 52 from the top and another cutting passsequence from the bottom an entire matrix of polygonal apertures A isformed, without the necessity of individually forming each aperture holeas required in known collimators 20, 30 or drilled tungsten collimators.

FIGS. 7-10 show an alternative embodiment two-dimensional aperture arraycollimator 60 that has a first plurality of septa slats 62 oriented in afirst direction and a second plurality of septa slats 64 that aremachined in the same material block. Slats 62 and slats 64 are orientedin skewed (here orthogonal) relationship to each other, thus definingrespective aperture channels 61, 63 that when combined form a matrix orgrid of two-dimensional rectangular or square polygonal cross-sectionapertures A. The aperture A profile may be selectively varied bychanging respective slat 62, 64 thickness and spacing. As one skilled inthe art can appreciate the two-dimensional aperture collimator 60 may beformed by cutting one side of septa slats 62 or 64, as shown in FIG. 6,then cutting the opposite side so that the apertures on both sides arein open communication. The cross-sectional polygonal profile of thecombined two-dimensional apertures A (e.g., square or rectangular)establishes the photon collimation profile that enables passage ofphotons having trajectories in alignment with the aperture andattenuates photons that do not have an aligned trajectory. Thus, with aslittle as two continuous EDM or laser ablation tool cutting passes (or arelatively simple pair of skewed laser sintered arrays), a grid ofthousands of apertures A may be formed quickly with the high precisionrequired for collimator applications. It would be considerably more timeconsuming and expensive to cut or sinter thousands of individualapertures in a single collimator by thermal ablation (e.g., laser orEDM) or laser sintering from alloy powders, while maintaining cuttingand grid alignment tolerances.

FIGS. 11-12 show a variable-focus collimator 70 that focuses photonsalong lines at the same or different focal lines, or focal lines thatvary symmetrically across the collimator. The variable-focus collimator70 has a first focal collimator having first and second pluralities ofintegrally formed septa slats 72, 74 respectively oriented in first andsecond opposed pattern arrays, wherein the first and second patternarrays are skewed in converging relative alignment with each other; andrespective pluralities of first and second aperture channels 71, 73between each pair of opposing slats 72, 74 that collectively define afocusing beam path.

The variable-focus collimator 70 also has a second focal collimator onan opposite side of the solid block from the first and second arrays 72,74, having third and fourth pluralities of integrally formed septa slats76, 78, respectively oriented in third and fourth opposed patternarrays, wherein the third and fourth pattern arrays are skewed inconverging relative alignment with each other. Respective pluralities ofthird and fourth aperture channels 75, 77 are formed between each pairof opposing slats 76, 78. The respective first and second pluralities ofslats pattern arrays 72, 74 are generally orthogonally aligned relativeto the third and fourth pluralities of slats 76, 78 pattern arrays andform polygonal cross-section through-apertures. While the embodiment inFIGS. 11 and 12 shows a focusing collimator, one skilled in the art canappreciate that the septa slats can be constructed in any profile andarrays of slats can be oriented in any relative position necessary todirect the photon beams. Thus one may construct a focusing or divergingtrajectory photon collimator as well as a parallel trajectory photoncollimator. More particularly, the respective first and secondvariable-focus collimators may each focus to a line at the same focaldistance (cone beam), to lines at different focal distance (astigmaticcone beam), or each may have focal lines which are variable across thecollimator array surface.

Collimator arrays of septa slats may be stacked and affixed relative toeach other in order to create a stacked collimator assembly withplanform projection apertures of any desired profile. As shown in FIGS.13 and 14, a stacked collimator 80 comprises three tungsten blocks 82,84, 86 having septa slats are stacked and affixed relative to each otherin order to create triangular profile apertures A interspersed withinthe grid pattern. Other polygonal aperture shapes, such as rectangles,squares, octagons diamonds, and triangles may be formed by stacking andskewing collimator arrays, but will impact the “packing” density andefficiency of the collimator. Variances in aperture shapes interspersedwithin the grid pattern that may arise from utilization of stacked slatsare compensated for during collimator calibration.

Referring to FIG. 15, collimator 90 has septa slat 92 profiles thatinclude slat apertures 93 formed therein during the slat cutting processas the EDM wire or laser cutting head translates along the length L ofthe slat. The slat apertures 93 may be formed in other polygonalprofiles. In this exemplary embodiment the slats 92 are affixed to rails98, similar to the embodiment of FIGS. 3-5. The collimator 90 structuremay also be fabricated using laser sintering methods.

Although various embodiments which incorporate the teachings of thepresent invention have been shown and described in detail herein, thoseskilled in the art can readily devise many other varied embodiments thatstill incorporate these teachings. For example collimator apertures andsepta slats may be constructed with other polygonal profiles andoriented in other desired relative spacing in order to achieve desiredphoton beam orientation. The collimator embodiments shown herein may befabricated from alloys other than tungsten or molybdenum, includinglead.

What is claimed is:
 1. A method for making an photon collimator,comprising cutting a plurality of elongated aperture channels through ablock of photon-attenuating material by thermal ablation machiningselected from the group consisting of electric discharge machining andlaser machining, the respective channels having an elongated lengthdimension greater than width and depth dimensions and oriented in apattern array that is laterally spaced relative to their respectivewidth dimensions, thereby leaving a pattern of opposed septa slatsdefining width and length dimensional boundaries of said aperturechannels.
 2. The method of claim 1 further comprising: cutting a firstplurality of parallel aperture channels having a first depth partiallythrough one side of the block; cutting a second plurality of parallelaperture channels having a second depth partially through an oppositeside of the block; and the respective first and second channels being inskewed relative alignment and in communication with each other, forminga pattern of polygonal cross-section through apertures through theblock.
 3. The method of claim 2, wherein the respective first and secondchannels are orthogonally aligned relative to each other and formrectangular cross-section through-apertures.
 4. The method of claim 3,wherein the respective first and second channels are equally spacedrelative to each other and form square cross-section through-apertures.5. The method of claim 1 further comprising: forming a stackedcollimator assembly by stacking a plurality of said collimator blocks oneach other, so that respective aperture channels in each block are incommunication with each other and define through-apertures passing fromone side of the block stack through an opposite side of the block stack;skewing relative alignment of respective aperture channels in eachblock, thereby forming polygonal cross-section through-apertures; andaffixing the blocks to each other.
 6. The method of claim 1, wherein thecutting step further comprises passing an EDM wire cutting head alongthe pattern array, repeating sequential cutting of respective channeldepth and width.
 7. The method of claim 2, wherein the cutting stepscomprise a first passing of an EDM wire cutting head along the firstplurality of channels pattern array, repeating sequential cutting ofrespective first channel depth and width; and a second passing of an EDMwire cutting head along the second plurality of channels pattern array,repeating sequential cutting of respective second channel depth andwidth.
 8. A photon collimator, comprising a block of photon-attenuatingmaterial defining a plurality of integrally formed septa slats, eachslat having an elongated length dimension greater than its thickness anddepth dimensions, and oriented in an opposed pattern array that islaterally spaced relative to its thickness dimension; and an aperturechannel defined between each pair of opposed slats.
 9. The collimator ofclaim 8, further comprising: first and second pluralities of integrallyformed parallel planar septa slats on opposite sides of the block, thatare skewed in relative alignment with each other; and respective firstand second aperture channels between each pair of opposing slats incommunication with each other and forming a pattern of polygonalcross-section through-apertures.
 10. The collimator of claim 9, whereinthe respective first and second pluralities of slats are orthogonallyaligned relative to each other and form rectangular cross-sectionthrough-apertures.
 11. The collimator of claim 10, wherein therespective first and second pluralities of slats are equally spacedrelative to each other and form square cross-section through-apertures.12. The collimator of claim 8, further comprising: a firstvariable-focus collimator having first and second pluralities ofintegrally formed septa slats respectively oriented in first and secondopposed pattern arrays, wherein the first and second pattern arrays areskewed in converging relative alignment with each other; and respectivepluralities of first and second aperture channels between each pair ofopposing slats that collectively define a photon path.
 13. Thecollimator of claim 12, further comprising: a second variable-focuscollimator on an opposite side of the block from the first and secondarrays, having third and fourth pluralities of integrally formed septaslats, respectively oriented in third and fourth opposed pattern arrays,wherein the third and fourth pattern arrays are skewed in convergingrelative alignment with each other; respective pluralities of third andfourth aperture channels between each pair of opposing slats; andwherein the respective first and second pluralities of slats patternarrays are generally orthogonally aligned relative to the third andfourth pluralities of slats pattern arrays and form polygonalcross-section through-apertures.
 14. The collimator of claim 13, formingwherein the respective first and second variable-focus collimators areconstructed to form a collimator selected from the group consisting ofcone beam, astigmatic beam or other symmetric beam collimators.
 15. Thecollimator of claim 14, wherein the respective first and secondpluralities of slats pattern arrays are orthogonally aligned relative tothe respective third and fourth pluralities of slats pattern arrays andform square cross-section through-apertures.
 16. The collimator of claim8 further comprising: a stacked collimator assembly having a pluralityof said collimator blocks stacked on and affixed to each other so thatrespective aperture channels in each block are in communication witheach other and define through-apertures passing from one side of theblock stack through an opposite side of the block stack; and therespective aperture channels in each block in skewed relative alignment,thereby forming polygonal cross-section through-apertures.
 17. Thecollimator of claim 16 comprising three blocks having respectiveaperture channels in skewed relative alignment forming triangularcross-section through-apertures.
 18. A photon collimator, formed by themethod of claim
 1. 19. A photon collimator, formed by the method ofclaim
 2. 20. A photon collimator, comprising: a block ofphoton-attenuating material defining first and second pluralities ofintegrally formed planar septa slats on opposite sides of the block,that are skewed in relative alignment with each other; each slat in eachrespective plurality having an elongated length dimension greater thanits thickness and depth dimensions, and oriented in an opposed patternarray that is laterally spaced relative to its thickness dimension; andrespective first and second aperture channels between each correspondingpair of respective opposing first and second pluralities of slats; therespective first and second aperture channels in communication with eachother and forming a pattern of polygonal cross-sectionthrough-apertures.